Lately a turbo code or a low density parity check code (LDPC code) are used not only in a communication or broadcasting field, but also in a field of recording-regenerating media of digital data, such as magnetic disks or optical disks. It is known that the turbo code or the LDPC code has an error correction capability close to a theoretical limitation, and a recording-regenerating apparatus that handles reliability information as a soft decision value has been proposed (e.g. see Patent Document 1).
FIG. 11 is a diagram depicting a configuration of a conventional recording-regenerating apparatus that handles a soft decision value. In FIG. 11, the recording-regenerating apparatus 101 includes an encoder 102, a PR (Partial Response) transmission path 103, and a decoder 104.
The encoder 102 includes an error correction encoder 111 and a modulation encoder 112. The error correction encoder 111 generates an error correction code sequence by attaching a parity sequence to an input data sequence based on a predetermined rule. The modulation encoder 112 encodes the input error correction code sequence into a predetermined modulation code based on a predetermined modulation rule, and outputs the modulation code, to which a predetermined restriction is added, to the PR transmission path 103 as an encoded sequence.
For example, a DC free restriction which equalizes a number of codes “0” and a number of codes “1” in a sufficiently long range, a (d, k) restriction which makes a minimum length and a maximum length of a number of “0s” that continue to d and k respectively, or the like is used as the predetermined restriction.
The PR transmission path 103 includes a recording-regenerating portion 113 and an equalization processor 114. The PR transmission path 103 performs recording process or regenerating process in a recording-regenerating channel of PR2 (Partial Response class-2).
The recording-regenerating portion 113 performs NRZI (Non-Return to Zero Inverted) encoding on the encoded sequence input from the modulation encoder 112, and records the NRZI-encoded signal on the installed recording medium or internal recording medium using a mark edge recording method. The recording-regenerating portion 113 also reads out the encoded signal recorded on a recording medium via the PR2 channel, and supplies the read out and encoded signal to the equalization processor 114.
The equalization processor 114 performs PR equalization processing using waveform interference on the encoded signal supplied from the recording-regenerating portion 113, so as to have predetermined target equalization characteristics, and supplies the encoded signal, to which the PR equalization processing has been performed, to the decoder 104.
The decoder 104 includes a PR-SISO decoder 115, an SISO demodulator 116 and an error correction decoder 117. The PR-SISO decoder 115 performs predetermined decode processing on the encoded signal supplied from the equalization processor 114, and outputs a soft decision value. Here, SISO is an acronym for Soft-Input Soft-Output, and refers to the processing to input-output the soft decision value.
The PR-SISO decoder 115 determines trellis representation, where a state transition table that indicates the encoding process of each time is developed in a time series based on the NRZI encoded data and the PR2 channel, from the encoded signal received from the PR transmission path 103. The PR-SISO decoder 115 computes probability based on the determined trellis representation in the NRZI encoded data and the PR2 channel so as to calculate the reliability information as the soft decision value.
The SISO demodulator 116 calculates reliability information modulated and decoded using a trellis based on the modulation rule of the modulation encoder 112.
The reliability information calculation using the trellis by the PR-SISO decoder 115 and the SISO demodulator 116 is executed according to a BCJR (Bahl-Cocke-Jeinek-Raviv) algorithm, for example.
The error correction decoder 117 uses turbo decoding, for example. The error correction decoder 117 performs error correction by performing turbo decoding corresponding to the turbo code used by the error correction encoder 111. Patent Document 1 also discloses an example of performing error correction decoding using the LDPC code.
Patent Document 2 discloses a Sum-Product decoding method that implements decoding by the LDPC code (hereafter called “LDPC decoding”).
Non-patent Document 1 presents an example of soft decision decoding using a trellis based on a PR transmission path and a modulation rule in signal processing for optical disks.
Further, Viterbi decoding, used in signal processing of regenerative signal for optical disks and the like as PR decoding, is known.
When executing LDPC decoding, reliability information as a soft decision value is calculated by modeling, in general, signals modulated by binary phase reversal amplitude shift keying (PR-ASK), binary phase shift keying (BPSK), or the like, using an additive white Gaussian noise (AWGN) transmission path. In this specification, in order to simplify description, the AWGN transmission path in the BPSK modulation, which is most easily handled, is defined as a model of a general transmission path of the LDPC decoding. The PR-ASK modulation may be regarded as the equivalent to the BPSK modulation.
When applying the LDPC decoding to an actual transmission path such as communication and recording-regenerating, distribution of the reliability information calculated by the channel decoding and demodulation processing may sometimes deviate compared to distribution of the reliability information calculated by the LDPC decoding in which modeling is performed with the general AWGN transmission path in the BPSK modulation. The cause of the deviation of distribution is an influence of the modulation rule, in which the information of an arbitrary element (bit) of original data is dispersed into plural elements (bits) in the data after conversion, or an influence of the transmission path, in which the information of an arbitrary element (bit) of the original data is dispersed into plural components of the regenerative signal. In particular, the deviation of distribution, in which the reliability information corresponding to an error bit is distributed over a wide range, degrades the performance of the LDPC decoding when applied to an actual transmission path, compared with the performance of the LDPC decoding when modeled with the general AWGN transmission path in the BPSK modulation.
The deviation of distribution of the reliability information like this may be checked by simple simulation using a BCJR algorithm based on PR (12221) as the equalization system, for example. In this simulation, the deviation of the distribution of the reliability information in particular notably appears as a spread of the distribution of a portion corresponding to an error bit.
By the dispersion of the information in the modulation or the transmission path, the distance in the original data and the distance in the modulated data or in the signal on the transmission path become different values. As a result, an error generated by noise on the transmission path may influence the decoding of the original data as major errors.
When the reliability information of a bit is calculated from the modulated data or a signal on the transmission path, where information of the bit of the original data has been dispersed, the dispersion of information of the bit is not considered in the case of conventional PR decoding using a BCJR algorithm, disclosed in Patent Document 1, for example. Therefore the dispersed information is not efficiently reflected on the reliability information of the bit. As a result, it is difficult to demonstrate the original decoding performance in conventional PR decoding, such as the LDPC decoding, which utilizes this reliability information of the bit.    Patent Document 1: Japanese Patent Application Laid-Open No. 2005-141887    Patent Document 2: Japanese Patent Application Laid-Open No. 2007-272973    Non-patent Document 1: Eiji Yamada, Tetsuo Iwaki and Takeshi Yamaguchi, “Turbo Decoding with Run Length Limited Code for Optical Storage”, Japanese journal of applied physics, March 2002, Vol. 41 (2002), pp. 1753-1756, Part 1, No. 3B